Field of the Invention
The present invention relates to methods, apparatus and computer programs for transmitting and/or receiving signals within an electric power grid.
Description of the Related Technology
Communication systems for transmitting and/or receiving data across an electric power grid are known. Transmitting data across an electric power grid is advantageous because it uses existing infrastructure as the transmission medium. Since most premises that need a communications link are already connected to the electric power grid, a user may connect to the communications link without any additional infrastructure being required. Furthermore, the number of users already connected to the electric power grid, and the number of points at which one can connect to the electric power grid, is large, and therefore the infrastructure of the electric power grid provides a flexible network over which communication can be established.
Typically, in such a communication system, a data signal comprising a modulated carrier wave is superimposed onto an AC power signal. Typically, the superimposed data signal is modulated at a frequency much higher than a grid frequency of the electric power grid. For example, a data signal in the frequency range kHz to MHz may be superimposed on a mains signal with a grid frequency of nominally 50 or 60 Hz.
Such methods for transmitting and/or receiving data within an electric power grid are often limited in range because conventional power lines that carry the AC power signal and therefore carry the data signal are designed to be efficient at mains frequencies and tend to attenuate frequencies higher than the grid frequency. Furthermore, other components in the infrastructure of the electric power grid, such as transformers, are also designed to operate at mains frequencies and consequently do not transmit higher frequency signals efficiently. Therefore, it is usually necessary to provide a route around such electric power grid components to prevent loss in signal strength and/or reduction in quality of the data signals. This adds cost and complexity to a communications system that is intended to make use of existing infrastructure.
Systems causing variations in the fundamental frequency of the supply, which can be detected, are also known. Such systems typically control the rotational speed of turbines that are supplying power to the grid in order to cause a change to the nominal frequency. Due to the inertia of the so-called “spinning generation” changes to a nominal frequency of the grid are relatively slow (in the order of tens of seconds). Furthermore, the frequency variations must be readily distinguishable from background variations from the nominal grid frequency that are typical in electric power grids (referred to hereinafter as noise), which typically requires the fundamental frequency to be varied by 100 mHz or more. This reduces the number of available frequency states because grid operators are obliged to control frequency to within specified limits (typically within a few percent of the nominal grid frequency) and states falling outside those limits will trigger a frequency stabilizing response, such as load shedding or connection of generation reserves.
It is an object of the present invention to at least mitigate some of the problems of the prior art.